1. Field of the Invention
The present invention generally relates to testing and characterization of lenses and, more particularly, to the quantitative measurement of radially asymmetric aberrations in lithographic lenses.
2. Description of the Prior Art
Lithography is used in numerous manufacturing processes, especially where complex patterning for selective processing at a surface is required. For that reason, lithography is of extreme importance in the manufacture of semiconductor devices and high density integrated circuits, in particular. High density of integration yields both functional and economic advantages in integrated circuits since close spacing of components reduces signal propagation time and increases noise immunity while allowing more chips of increased functionality to be produced by a single iteration of a series of process steps on each wafer. Integration density has also been enhanced by the exploitation of various self-aligned processes with can develop features and structures of smaller size than can be produced lithographically. However, self-aligned processes rely on the prior patterning of fine features produced through lithographic processes.
While many different lithographic processes and variations thereon are known, all share the common features of covering a surface with a resist which is sensitive to a particular form of energy or incident particles, selectively exposing portions of the resist to a dose of that energy or such particles sufficient to cause differential chemical response to a developer which removes either exposed or unexposed portions of the resist (depending on whether the resist is of a positive or negative type) and performing a process (generally deposition, etching or implantation) differentially on the exposed and unexposed portions of the surface.
Lithographic processes, however, at the feature sizes required by modern integrated circuits, are subject to severe and unavoidable physical limitations related to how the exposure is made. Specifically, for radiant energy, the minimum feature size which can be obtained is related to the wavelength of the energy used for the exposure. While X-rays have been investigated and used for lithographic exposures, the nature of the required mask and difficulty of fabrication thereof at the present state of the art renders X-rays generally impractical for semiconductor fabrication and charged particle beam tools are often used when extremely small feature sizes must be obtained. However, throughput of charged particle beam tools is relatively low and optical exposure at short wavelengths (e.g. deep ultra-violet) remains the exposure medium of choice for most integrated circuit manufacture.
Optical lithography techniques are relatively well-developed and numerous devices and techniques, such as the use of phase-shift masks or reticles using interference effects to adjust exposures and doses, have been developed to enhance exposure images and reduce the minimum feature size that can be produced with light of a given wavelength. However, aberrations (e.g. localized shift of focus or departure from planarity of the best focussed image) in the projection lens system have become increasingly critical and, at the current state of the art, are a fundamental limitation on exposure tool performance. While some precorrections may be made if the aberrations are well-understood, such information is difficult to obtain with suitable accuracy for this purpose and seldom supplied by the lens manufacturer.
Additionally, lens aberrations may change over time and as-manufactured aberration measurements may become obsolete. Changes in aberrations over time may possibly cause anomalous effects in combination with any form of precorrection. Further, very slight changes in lens shape or relative location become relatively more significant at very short wavelengths. Therefore, at the level of performance now required of lens systems for optical lithography, a robust in-situ method for simultaneous measurement of coma, astigmatism and higher order aberrations is needed to support maintenance of exposure tool performance.
Unfortunately, no suitable measurement method has been available and even test bench arrangements at the present state of the art do not support the required accuracy of measurement or permit accurate characterization of the aberrations which may be present. In particular, such systems generally cannot detect some aberrations such as coma when the illumination in the lens is radially symmetrical. Coma and other aberrations (e.g. focus, astigmatism) have required the use of reticles specifically designed to detect a particular form of aberration. Therefore, separate tests are required for each aberration and the sequence of tests is not practical for a production environment due to the increased testing time required for separate tests. Further, separate tests may also be a source of error in measurement and, in any event, do not cover a full set of azimuthal aberrations (e.g. non-radially symmetrical aberrations including coma, so-called three-leaf and four-leaf clover and the like), much less simultaneously, or otherwise provide a capability of minimizing non-productive time of the lithography tool.